The invention relates to the general field of image distortion due to Optical Proximity Effects with particular reference to ways to correct for this effect.
The proximity effect is a form of optical distortion associated with photoresist images. For a given development time, whether or not a given area of a photoresist layer will be left or removed after the development process depends on the total amount of energy deposited in that area during its exposure to radiation. Image features whose size and/or separation approach the resolution limit of said radiation will thus be subject to distortion that depends on how the diffraction maxima and minima, that lie on both sides of a xe2x80x98sharpxe2x80x99 edge, interact with one another.
The proximity effect can be compensated for, at least in part, by modifying any given feature in the opposite direction to the expected distortion. Thus, a line that would otherwise come out too narrow can be drawn as wider than its true width, etc. The data that represents the information from which a mask suitable for use in photolithography can be generated, is stored in a data file so corrections to allow for the proximity effect will also be stored there. The overall nature and scope of these corrections, and how they get into the file, will vary with the application and the user.
The optical proximity correction (OPC) is commonly calculated by summing two Gaussian functions whose value depend on a critical dimension (CD) defined by the design rules as well as on the wave-length of the exposing radiation. In general, the distortion of lines that are part of a dense assemblage will be more positive than the distortion of isolated lines in optical mode. While OPCs obtained in this manner provide satisfactory results, the computation time can be very long, typically about 16 hours for a single mask file using state of the art computers.
An examination of the changes made to mask images as a result of applying OPC, shows that the OPC takes two princial formsxe2x80x94scatter bars and serifs. The present invention is concerned only with the latter. A serif is a small square that is added to the corner, or vertex, of a stripe. Vertices may be positive or negative, corresponding to whether they are convex or concave. A positive serif extends the boundaries of a positive vertex while a negative serif reduces the boundaries of a negative vertex.
Referring now to FIG. 1a, an example of a pattern of stripes is shown such as might form part of a layout mask for an integrated circuit. FIG. 1b shows the photoresist image that is obtained from the pattern of FIG. 1b in the absence of any OPC. Several distortions of the original pattern can be seen to have taken place. The stripes have thickened at their ends (where they are not close to any other stripe). Thus dimension 3 of stripe 2 (for example) can be seen to be less than dimension 13 of stripe 12. Similarly, the part of stripe 5 that is not near other stripes has been thickened in stripe 15. It can also be seen that the lengths of stripes 12 and 15 is less than the lengths of stripes 2 and 5.
Another important form of distortion can be seen to have occurred at positive vertex 4 and negative vertex 6. Vertex 14 has been truncated, giving the corner a rounded appearance while at vertex 16 just the opposite has occurred and the strip boundary can be seen to have cut the corner. Finally, the ends of all stripes in FIG. 1b can be seen to be rounded, even though they are quite square in FIG. 1a. 
FIG. 2a shows the pattern of FIG. 1a after it has received the full OPC treatment. Of interest in the present context are the serifs that have been added to the pattern. Examples are positive serif 24 that has been added at vertex 4 and negative serif 26 that has been added at vertex 6. Note also the hammer-heads that have been added to the line ends (for example at 22 or 25). Such hammerheads are the result of the merger of the two serifs that were added separately to the two vertices located at the stripe""s end.
FIG. 2b shows the photoresist pattern obtained as a result of exposing through a mask containing the pattern of FIG. 2a. It is readily seen that the pattern in FIG. 2b is very close to the original pattern (FIG. 1a).
As already noted, the cost of computing a full OPC as exemplified in FIG. 2a, is very high. Since it is apparent that much of the distortion due to proximity effects can be removed by the appropriate placement of serifs, a method of OPC that adds only serifs might be as effective as the full OPC treatment, provided that the distortions that have not been corrected do not introduce shorts, opens, hot spots, etc. in the line patterns that end up being formed in the integrated circuit.
A number of approaches have been taken in the prior art to dealing with the proximity effect without the need to perform the full OPC calculation. Wampler et al. (U.S. Pat. No. 5,663,893 September 1997) describe a method of simplifying the Optical Proximity Correction by adding positive and negative scatter bars as well as positive and negative serifs. The main thrust of this invention is directed towards teaching a method for handling large data files that represent wiring layouts and no specifics are provided as to how the serifs are incorporated in the line drawings. No information is presented as to what cost savings, if any, their methodology provides.
Liebmann (U.S. Pat. No. 5,553,273 September 1996) aims to correct Optical Proximity Effects by biassing critical portions of the design. In particular, this invention attempts to minimize the creation of new vertices so that it actually teaches away from the practice of using serifs.
Liebmann et al. (U.S. Pat. No. 5,657,235 August 1997) use the OPC data to drive the mask writer itself rather than changing the data design file. By assigning relative mask writer doses, as needed, they are able to bring about continuous line width variations (to compensate for OP effects) without increasing the size of the data design file. Serifs and scatter bars are not involved.
It has been an object of the present invention to provide a method for correcting distortions due to the Optical Proximity Effects.
Another object of the invention has been that said method require the use of very little computer time, particularly when compared to existing methods for computing Optical Proximity Corrections.
Yet another object has been that said method not require more computer memory than is used by the existing methods.
These objects have been achieved by limiting correction of the optical mask to the addition of two sets of serifs (one larger than the other) at the appropriate vertices in the layout mask. A key feature of the method is that identification of which serifs will be within a critical distance from a neighbouring edge (and have therefore to be smaller) is performed by the application of a few simple logical operations. This results in a corrected mask that can be generated in a few minutes, as opposed to many hours.